I have to agree here with SFbay. I know my thermostat is good and coolant was circulating the way u DFEL750I describe since i bought a car.It takes 1 mile run from a cold start for thermostat to open. Big engine 7 has.

1 mile run?? Really?? I have 1000 doubts about it.
If you're saying its opened, how do you know?? Did you check lower hose?? I bet 1000$ it's going to be cold until your engine reaches 100-105C. Boil your tstat and you'll see that it starts working only from 100C. Before that its fully closed. Ppl who have more than 105C would notice that lower hose is cold even at 105C therefore aux fan won't kick in when needed.